1. Field of the Invention
This invention relates to a laser diode array, and more particularly to a laser diode array comprising a plurality of multi-cavity laser diode chips, each having a plurality of light emitting points, fixed side by side.
This invention further relates to a laser device, a wave-coupling laser source and an exposure device using such a laser diode array.
2. Description of the Related Art
As devices for generating a laser beam in an ultraviolet region, there have been put into practice, for instance, a wavelength conversion laser in which infrared light generated from a semiconductor-laser-pumped solid state laser is converted to its third harmonic in an ultraviolet region, an excimer laser, and an Ar-laser.
Further, GaN-semiconductor lasers radiating a laser beam substantially at 400 nm have been recently provided as shown in “Jpn. Appl. phys. Lett.”, Vol. 37, p. L1020.
A light source radiating a laser beam whose wavelength is in such a wavelength range is useful as an exposure light source for exposing a photosensitive material having a sensitivity to a certain wavelength in an ultraviolet region (a wavelength region including 350 to 420 nm). Needless to say, such an exposure light source should have an output power sufficient to cause the photosensitive material to react.
In order to use as the exposure light source, the excimer laser is too large in size, which increases the manufacturing cost and the maintenance cost.
The wavelength conversion laser in which infrared light is converted to its third harmonic is very difficult to obtain high output power due to its very poor wavelength-conversion efficiency. At present it is practical to pump a solid laser medium by a semiconductor laser of 30 W to cause a fundamental wave (1064 nm in wavelength) of 10 W to oscillate and to convert the fundamental wave to its second harmonic (532 nm in wavelength), thereby obtaining a third harmonic (355 nm in wavelength) of 1 W as the sum frequency of the fundamental wave and its second harmonic. The power-light conversion efficiency of the semiconductor laser is only 50% in this case, and the conversion efficiency to ultraviolet light is as low as about 1.7%. Further, use of an expensive wavelength conversion element adds to the cost of such a wavelength conversion laser.
Further, use of the Ar-laser encounters a difficulty that the Ar-laser is as low as 0.005% in power-light conversion efficiency and is as short as about 1000 hours in service life.
In the GaN-semiconductor laser, since a low-transition GaN-crystal substrate cannot be obtained, there have been made attempts of obtaining high output power and high reliability by making a low-transition region of about 5 μm by a method of growth called FLOG and forming a laser region thereon. However, even in a GaN-semiconductor laser thus obtained, those which are as high as 500 mW to 1 W have not been commercially available because of the difficulties in making a low-transition substrate in a large area.
Another attempt of obtaining a high output power semiconductor laser involves obtaining 10 W output power by forming, for instance, 100 cavities each emitting light of 100 mW, and coupling the outputs of the cavities. However, to form cavities of as many as 100 at high yield is hardly practical. Especially, it is difficult to make a GaN-semiconductor laser at a yield higher than 99% even if it is of a single cavity.